Zn Diffusion and α-Fe(Zn) Layer Growth During Annealing of Zn-Coated B Steel

Direct hot press forming of Zn-coated 22MnB5 steels is impeded by micro-cracks that occur in the substrate due to the presence of Zn during the forming process. A study was therefore undertaken to quantify concentration of Zn across the alpha-Fe(Zn) coating and on grain boundaries in the alpha-Fe(Zn) layer and the underlying gamma-Fe(Zn) substrate after isothermal annealing of Zn-coated 22MnB5 at 1173 K (900 A degrees C) and to link the Zn distribution to the amount and type of micro-cracks observed in deformed samples. Finite difference model was developed to describe Zn diffusion and the growth of the alpha-Fe(Zn) layer. The penetration of Zn into the gamma-Fe(Zn) substrate after 600 seconds annealing at 1173 K (900 A degrees C) through bulk diffusion is estimated to be 3 mu m, and the diffusion depth of Zn on the gamma-Fe(Zn) grain boundaries is estimated to be 6 mu m, which is significantly shorter than the maximum length (15 to 50 mu m) of the micro-cracks formed in the severely stressed conditions, indicating that the Zn diffusion into the gamma-Fe(Zn) from the alpha-Fe(Zn) during annealing is not correlated to the depth of micro-cracks. On the other hand, the maximum amount of Zn present in alpha-Fe(Zn) layer decreases with annealing time as the layer grows and Zn oxidizes, and the amount of Zn-enriched areas inside the alpha-Fe(Zn) layer is reduced leading to reduced length of cracking. Solid-Metal-Induced Embrittlement mechanism is proposed to explain the benefit of extended annealing on reduced depth of micro-crack penetration into the gamma-Fe(Zn) substrate.